Nowadays, from the viewpoint of a decrease in environmental load and cost, it is desirable to reduce the amount of use of steel sheets which are used for cans for food and beverages. Accordingly, the thickness of steel sheets is being reduced regardless of whether the steel sheet is used for two-piece cans or three-piece cans. However, problems due to a decrease in the thickness of steel sheets are recognized. For example, can bodies are deformed due to an external force which is applied when the cans are handled in a can manufacturing process, in a transporting process and in the market and can bodies are deformed (buckled) due to a change in the external pressure of the cans which occurs when a heat sterilization treatment for contents is performed.
Heretofore, the strength of steel sheets has been increased in order to increase the resistance to deformation described above. However, increasing the strength of steel sheets causes a problem in the can manufacturing process, because there is an increase in deformation resistance and heat generation due to working when two-piece cans are formed by performing DI (Draw and wall Ironing) forming or deep drawing and ironing forming. In addition, an increase in the strength of a steel sheet causes an increase in the rate of occurrence of neck wrinkles and flange cracks when neck forming is performed after forming of a can body has been performed and when flange forming is performed thereafter. As described above, increasing the strength of steel sheets is not necessarily an appropriate method for compensating for a decrease in resistance to deformation due to a decrease in the thickness of steel sheets.
On the other hand, the buckling phenomenon of can bodies occurs due to a decrease in the rigidity of the can body caused by a decrease in the thickness of the can bodies. Therefore, in order to increase resistance to buckling (also called paneling strength), it is thought to be effective as a method to optimize the size and design of a can body for increasing the rigidity of the can body.
In addition, it is thought to be effective to increase rigidity by increasing the Young's modulus of a steel sheet. There is a strong correlation between the Young's modulus and crystal orientation of steel. A crystal orientation group (α fibers), in which the <110> orientation is parallel to the rolling direction, increases a Young's modulus in the width direction which is at 90° with respect to the rolling direction, and it is theoretically possible to form a steel sheet having a Young's modulus of about 280 GPa by increasing the integrated intensity of, in particular, the {112}<110> orientation. In addition, a crystal orientation group (γ fibers), in which the <111> orientation is parallel to the normal direction of a sheet surface, can increase the Young's moduli in directions at angles of 0°, 45°, and 90° with respect to the rolling direction up to about 230 GPa. On the other hand, in the case where a crystal orientation is not integrated in a particular direction in the steel sheet, that is, a steel sheet has a random texture, the Young's modulus of the steel sheet is about 205 GPa.
Many steel sheets have been provided focusing on a high Young's modulus in order to compensate for a decrease in the rigidity of vehicle bodies due to a decrease in the thickness of steel sheets to be used for automobiles.
For example, Patent Literature 1 discloses a technique for increasing the Young's modulus in a direction at 90° with respect to the rolling direction, the technique including using ultralow-carbon steel containing Nb or Ti, forming a ferritic texture in which the {311}<011> and {332}<113> orientations are accumulated at the hot rolled steel sheet stage by promoting a transformation from a non-recrystallized austenite phase to a ferrite phase in the hot rolling process under the condition that the rolling reduction ratio at a temperature in the range of the Ar3 point to (the Ar3 point+150° C.) is 85% or more, and reforming the original texture into a texture in which the {211}<110> orientation is the primary orientation by performing cold rolling and recrystallization annealing.
In addition, Patent Literature 2 discloses a method for manufacturing a hot-rolled steel sheet having an increased Young's modulus in a direction at 90° with respect to the rolling direction, the method including growing {211}<110> by adding Nb, Mo, and B to a low-carbon steel containing, by mass %, 0.02% to 0.15% of C and by performing hot rolling under the condition that the rolling reduction ratio at a rolling temperature in the range of the Ar3 point to 950° C. is 50% or more.
On the other hand, methods for manufacturing a steel sheet focusing on the high Young's modulus of a steel sheet to be used for a can have been provided for a three-piece can.
Patent Literature 3 discloses a technique for manufacturing a steel sheet to be used for a container having an increased Young's modulus in a direction at 90° with respect to the rolling direction, the method including forming a strong rolled texture, that is, α fibers, by performing second cold rolling under the condition that the rolling reduction ratio is 50% or more, after performing cold rolling and annealing.
Patent Literature 4 discloses a method, without performing annealing, for manufacturing a steel sheet to be used for a container having an increased Young's modulus in a direction at 90° with respect to the rolling direction, the method including forming a strong a fibers by performing cold rolling on a hot-rolled steel sheet under the condition that the rolling reduction ratio is 60% or more.
In addition, Patent Literature 5 discloses a method for manufacturing a steel sheet to be used for a container having an increased Young's modulus in a direction at 90° with respect to the rolling direction, the method including adding Ti, Nb, Zr, and B to an ultralow-carbon steel, performing hot rolling under the condition that the rolling reduction ratio at a temperature equal to or lower than the Ar3 point is at least 50% or more, and performing annealing, after performing cold rolling, at a temperature of 400° C. or higher and equal to or lower than the recrystallization temperature.
On the other hand, in the case of a two-piece can which is manufactured by performing DI forming or deep drawing and ironing forming, there is a marked unevenness in body height at the opening of the formed can, which is called earring, and there is a decrease in yield in the case where the degree of earring is large. There is a problem in that anisotropy (Δr) in the steel sheet plane has to be decreased in order to prevent earring. Moreover, in the case where a laminated steel sheet is formed by a method of manufacturing cans such as DI forming or deep drawing and ironing described above, there is also a problem in that corrosion resistance may decrease due to the delamination of the coating film from the steel sheet which is a base metal after forming a can. That is to say, it is an important factor for a steel sheet, which is to be used as a base metal, to have excellent surface quality so that it does not have a rough surface in order to maintain good adhesiveness with the film even after forming that involves a high degree of working such as deep drawing or ironing has been performed.
In order to solve the problem described above, Patent Literature 6 discloses a steel sheet having good formability and no rough surface and a method for manufacturing the steel sheet, the method including effectively forming the microstructure of a hot-rolled steel sheet and a final product steel sheet to be used for a can so that the microstructure has a small uniform grain size by performing hot rough rolling on ultralow-carbon steel under the conditions that the total rolling reduction ratio is 80% or more and the reduction ratio of the final pass is 20% or more and by ending hot finish rolling under the conditions that reverse transformation due to the generated heat in the rolling occurs when the hot-rolled steel sheet goes through any one of the rolling stands in the finish rolling mill line, so that the finish rolling temperature may be equal to or higher than the Ar3−50° C.
Patent Literature 7 discloses a steel sheet to be used for a two-piece can suppressing the occurrence of earring and having good resistance to surface roughening after press forming has been performed and a method for manufacturing the steel sheet, the method including forming a microstructure after performing hot rolling so that the microstructure has equiaxed crystal grains and small uniform grain size by properly controlling hot rolling conditions such as cooling conditions after performing finish rolling and forming a microstructure having small uniform equiaxed grains after performing annealing so that the steel sheet has a Δr of −0.2 or more and 0.2 or less by maintaining the effects of the hot rolling until after performing cold rolling and annealing.
In addition, Patent Literature 8 discloses a steel sheet having good resistance to surface roughening and a method for manufacturing the steel sheet, the method including adding Nb to ultralow-carbon steel as base material and optimizing a pinning effect by controlling the amount and grain size of precipitates containing Nb to control the grain size of a ferrite phase to be as small as 6 μm to 10 μm.